01 - Feat/defer exercises (#91)

Author: kawpii

internal/exercises/catalog.yaml

@@ -134,6 +134,11 @@ concepts:
   test_regex: ".*"
   hints:
     - Define a custom error type and return it from a function.
+- slug: 28_defer
+  title: Deferred Execution
+  test_regex: ".*"
+  hints:
+    - Close the file using `defer` keyword with f.Close().
 
 projects:
 - slug: 101_text_analyzer

internal/exercises/solutions/28_defer/defer.go

@@ -0,0 +1,12 @@
+package deferr
+
+import (
+	"fmt"
+	"os"
+)
+
+func WriteToFile(*os.File) {
+	f := CreateFile()
+	defer f.Close()
+	fmt.Fprintln(f, "data")
+}

internal/exercises/templates/28_defer/defer.go

@@ -0,0 +1,11 @@
+package deferr
+
+import (
+	"fmt"
+	"os"
+)
+
+func WriteToFile(*os.File) {
+	f := CreateFile()
+	fmt.Fprintln(f, "ABC")
+}

internal/exercises/templates/28_defer/defer_test.go

@@ -0,0 +1,17 @@
+package deferr
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestClosingFileAfterWriting(t *testing.T) {
+	f := CreateFile()
+	WriteToFile(f)
+
+	// Trying to write to the file after closing
+	_, err := fmt.Fprintln(f, "123")
+	if err == nil {
+		t.Fatal("File wasn't closed!")
+	}
+}

internal/exercises/templates/28_defer/utils.go

@@ -0,0 +1,23 @@
+package deferr
+
+import (
+	"os"
+	"sync"
+)
+
+var (
+	fileInstance *os.File
+	once         sync.Once
+)
+
+// Create a singleton of a temporary file
+func CreateFile() *os.File {
+	once.Do(func() {
+		f, err := os.CreateTemp("", "tmpfile")
+		if err != nil {
+			panic(err)
+		}
+		fileInstance = f
+	})
+	return fileInstance
+}